In general, in an electrophotographic image forming apparatus, an exposure device exposes a surface of a photoreceptor to form an electrostatic latent image, a developing device develops the formed electrostatic latent image to form a toner image, and the formed toner image is transferred to a recording medium, so that an image is formed.
In the photoreceptor on which the electrostatic latent image is formed, irregularity of charging characteristics or sensitivity characteristics exists in an axial direction of a drum surface due to factors on manufacturing. Furthermore, when the toner image is formed by developing the electrostatic latent image, there is a problem that density irregularity occurs in a main scanning direction (a drum shaft direction) due to an influence of variance of a gap with a developer carrier that supplies toner to the photoreceptor, variance of a light amount of an exposure device in the main scanning direction, and the like.
In order to solve such a problem, in an image forming apparatus disclosed in Patent Literature 1, test print images extending over the whole main scanning direction of a development area are printed, and an exposure amount is corrected at each position in the main scanning direction such that a density at each position of the printed test print image in the main scanning direction reaches a target density. In Patent Literature 1, by using two test print images having exposure amounts different from each other, densities of the two test print images are compared with each other at each position in the main scanning direction, so that the exposure amounts are corrected (see paragraph 0060 of Patent Literature 1).
Patent Literature 2 discloses an example of a correction method of an exposure amount. In this method, a plurality of test print images are formed on one sheet (recording medium) by changing exposure amounts in a stepped shape, and an image reading device reads densities of the formed test print images. Then, a change rate of an image density to a change in the exposure amount is calculated in a step section corresponding to a target density, and an exposure amount required for obtaining the target density is calculated by primary linear interpolation on the basis of the calculated change rate.